1. Field of the Invention
The invention relates to a control circuit for driving a gas discharge lamp, in particular a fluorescent lamp, having a controllable converter for converting a DC voltage to an AC voltage and having two feed lines, which are connected on the AC-voltage side to the converter, and between which the gas discharge lamp can be connected, an inductor, a first capacitance and a first controllable switching element being connected in series in the feed lines. The invention also relates to a method for driving a gas discharge lamp, in particular a fluorescent lamp, said lamp being driven for the purpose of producing different brightnesses with an AC voltage generated by a converter or with a DC voltage derived from the AC voltage via an inductor, a first capacitance and a controllable switching element, which are connected in series with the gas discharge lamp.
2. Discussion of the Prior Art
The use of an electronic ballast is known for starting and for operating a gas discharge lamp, in particular a fluorescent lamp, which is understood as being a gas discharge lamp which is coated on the inside with a fluorescent material. Such an electronic ballast comprises, in addition to a converter for converting a DC voltage to an AC voltage, an inductor and a capacitance, which are connected in series with the gas discharge lamp. The inductor and the capacitance form a series resonant circuit which is operated at resonance for starting purposes by means of the frequency of the AC voltage of the converter being adjusted. This results in a high voltage across the gas discharge lamp which ultimately allows the gas discharge lamp to restart. After starting, the impedance of the gas discharge lamp falls to its operational value, as a result of which an operating voltage is set at the lamp.
In order to dim the gas discharge lamp, the operating voltage can be influenced by means of a switchable switching element. This is possible, for example, by means of phase gating or by means of pulse width modulation. When the gas discharge lamp has been started, it can be operated both with an AC voltage and with a DC voltage.
In particular for applications in the aircraft industry, it is necessary for the control circuit to be light and cost-effective. In this regard, operation at operating frequencies which are as high as possible would be desirable since; as a result, the size of the inductor required can be reduced. A high operating frequency moreover provides the advantage of flicker-free operation of the gas discharge lamp. Brightness variations at a frequency of ≧100 Hz can no longer be resolved by the human eye.
In the case of the described operation of a gas discharge lamp with an electronic ballast, a certain size for the inductor is disadvantageously required, however, in order to still be able to generate the required starting voltage. In this case, the starting voltage even needs to be increased as the operating frequency increases since the gas discharge lamp has increasingly less time available for restarting as the operating frequency increases.